The present invention relates to electronic ballasts for lighting a discharge lamp by high-frequency power, and a lamp fixture using the ballast.
Conventional fluorescent lamp fixtures are generally realized such that an individually stored discharge lamp lighting device, e.g., a ballast, causes a preheating current to flow in a discharge lamp filament. The ballast applies a starting voltage after a preheating period to light the discharge lamp. The filament heating conditions during preheating are known to influence lamp life. Lighting (or starting) a lamp with insufficient heating (cold start) will result in a shorter filament life due to sputtering. Excessively heating the filament also causes emissive evaporation in the filament. This can also shorten filament life.
Japanese Unexamined Patent Publication No. 2002-56995 discloses a preheating power source for preheating a filament by a current source, a preheating control circuit for controlling an output of the preheating power source, and a filament voltage detection circuit. This disclosure proposes that an output of the preheating power source and a period of preheating time are changed to prevent the filament voltage from exceeding a predetermined voltage during the preheating period. It is therefore possible to ensure a reasonable lamp life.
A lamp voltage is detected to avoid end glow occurring in the lamp filaments and to prevent the lamp voltage from exceeding a predetermined value (as in Japanese Unexamined Patent Publication No. 2002-56995). However, different filament heating conditions are observed in the same type of lamps due to variations in the filament resistance value. Accordingly, a lamp filament may not be heated to allow an optimum emission start.
For example, a cathode characteristic (i.e. filament characteristic) of a FHP32 lamp includes a resistance value with variations of 9.0Ω to 16.0Ω when a filament current is 0.4 A. A preheating current is subjected to a constant current control in the conventional example. Due to filament variations, filament power can vary up to 1.8 times from nominal due to filament variations. Therefore, a different filament heating condition (i.e. electrode temperature) is observed in preheating, and insufficient heating (i.e. cold start) may occur depending on filament variations. Cold start becomes a cause of a shorter lamp life.
Moreover, a filament voltage in only one of two filaments has been conventionally detected, but two of the filaments do not necessarily have the same filament characteristics. In the case where a resistance value is made larger due to variations in two filaments arranged in the same lamp, end glow may possibly occur in one but not both of the filaments. In this case, lamp life cannot be necessarily ensured.
Each lamp may thus have a different life due to filament variations in the conventional example, wherein a non-lighting lamp needs to be replaced frequently at the end of a lamp life. Accordingly, maintenance costs are increased.
The conventional example is also accompanied by different preheating times due to filament variations, which causes a problem that each lamp is lit (or started) at different timing in the case of using a plurality of discharge lamp lighting devices in a same illumination space, making users feel a sense of incompatibility.
The conventional example which focuses attention on end glow being a discharge phenomenon among filaments monitors a filament voltage to prevent the voltage from reaching a fixed value or higher to avoid end glow as stated above, but an optimum heating state (or electrode temperature) for emission is not necessarily realized due to filament (or resistance value) variations in each lamp. It is because an optimum filament state for emission is merely defined by an electrode temperature.